Abstract
Enantiomerically pure 9-substituted 11-oxa[9]helicene derivatives have been synthesized through furan-ring formation by a nucleophilic cyclodehydration reaction of enantiomerically pure helical polycondensed 2,2′-diphenoquinone derivatives (1,1′-bibenzo[c ]phenanthrenylidene-2,2′-diones). (P )-2,2′-diphenoquinone derivatives afforded (P )-oxa[9]helicenes, whereas (M )-2,2′-diphenoquinone derivatives afforded the corresponding (M )-oxa[9]helicenes. Therefore, the ring-closing reaction afforded the corresponding enantiomerically pure products without decreasing the enantiomeric excess, and it proceeded stereospecifically with retention of the configuration. The thermal stability of an oxa[9]helicene was studied by determining the decrease in its enantiomeric excess at various temperatures, and its racemization barrier was found to be 165.6 kJ/mol.
Key words helical structures - oxahelicenes - O-heterocycles - racemization - cyclodehydration - asymmetric synthesis